The present invention relates to a butt joining method wherein two members are butt-joined by utilizing plastic deformation. The present invention also relates to a mechanical clinching apparatus used to carry out the butt joining method. Further, the present invention relates to a joined rod obtained by the butt joining method, and also relates to a method of manufacturing a cylinder apparatus.
One type of hydraulic shock absorber has a structure as shown in FIG. 9, by way of example. The hydraulic shock absorber includes an inner tube (cylinder) 2 having a piston 1 slidably fitted therein. The inner tube 2 is housed in an outer tube 3, one end of which is closed. A piston rod (rod) 4 is connected at one end thereof to the piston 1. The other end of the piston rod 4 extends to the outside through a rod guide 5 and a rod seal 6 that are fitted to the opening end portions of both the inner and outer tubes 2 and 3. A hydraulic fluid sealed in the inner tube 2 is passed through piston valves 7 provided on the piston 1 and also through a base valve 8 provided in the bottom of the outer tube 3 to generate damping force during extension stroke and compression stroke. A reservoir 9 is formed between the inner tube 2 and the outer tube 3 to compensate for a change in amount of hydraulic fluid in the inner tube 2 that corresponds to the amount by which the piston rod 4 enters or withdraws from the inner tube 2.
When this type of hydraulic shock absorber is used as a suspension system of a vehicle, a bolt-shaped member (mounting member) 10 is joined to the forward end of the piston rod 4 to serve as a mounting part that is to be secured to a vehicle body-side member, and an eye 11 is joined to the bottom of the outer tube 3 to serve as a mounting part that is to be secured to an axle-side member. In addition, a spring retainer 12 is joined to an intermediate portion of the outer tube 3 to retain a coil spring. To butt-join the bolt-shaped member 10 to the piston rod 4, projection welding (see a welded joint 13 in the figure) has heretofore been generally used.
In such a butt joining process using projection welding, welding conditions, such as welding current and welding pressure, need to be strictly controlled in order to ensure the desired joining strength. Particularly, when medium to high carbon steel is used as a material of the bolt-shaped member 10 from the need to ensure high joining strength, much more strict control is required for welding conditions because such carbon steel is inferior in weldability. Further, the piston rod 4 is subjected to metal plating beforehand. In this regard, fine cracks that may be present in the metallic deposit on the piston rod 4 are likely to be expanded by the thermal effect of the projection welding, causing problems in terms of corrosion resistance. In addition, spatters caused by welding deteriorate working environments.
Under these circumstances, Japanese Patent Application Publication No. 2003-225733 proposes a countermeasure in which a threaded piston rod is used that has an annular groove previously formed on a part thereof where the head 10a of the bolt-shaped member 10 (see FIG. 9) would otherwise be provided, and a collar prepared as a separate member is joined (fitted) into the annular groove by utilizing plastic flow, whereby the bolt-shaped member 10 is omitted. This technique can solve various problems associated with the above-described welding. The countermeasure disclosed in this JP publication suffers, however, from the following problems. Because a thread is formed on a long rod, rolling process cannot be used to form a thread thereon, but instead a machining process is unavoidably required for thread cutting, which causes an increase in the production cost of the piston rod itself. In addition, because the overall length of the piston rod increases by an amount corresponding to the length of the bolt-shaped member 10, the metal plating equipment needs to be altered, and the cost increases correspondingly.
Japanese Patent Application Publication No. Hei 7-280016 discloses a technique of joining a mounting member to a piston rod of a gas spring. According to this technique, a cup-shaped female part is provided on the mounting member, and an annular groove is provided on one end part of the piston rod. After the one end part (male part) of the piston rod has been fitted to the female part of the mounting member so as to be butted against it, the side wall of the female part is locally pushed into the annular groove by mechanical clinching technique, thereby joining together the mounting member and the piston rod. With this technique, however, clinch portions formed by pushing the side wall of the female part are merely discontinuously provided along one annular groove. Therefore, the joining technique is not satisfactory in terms of joining strength and cannot be applied as it is to a piston rod of a hydraulic shock absorber for a suspension system that requires the above-described high joining strength.
Meanwhile, Japanese Patent Application Publication No. 2002-81483 discloses a method of manufacturing a hydraulic shock absorber. According to this method, after a piston rod having a piston secured to one end thereof has been inserted into a cylinder, a rod seal for sealing the cylinder is pressed axially with a tubular pressing jig, and while doing so, an upper end portion of an outer tube is deformed radially inward with a staking jig to secure the rod seal so as to close the opening end of the cylinder. In this case, the pressing jig is adapted to press the rod seal with the piston rod inserted therein. Therefore, if a bolt-shaped member of a predetermined size has previously been butt-welded to the piston rod, the head of the bolt-shaped member is likely to interfere with the pressing jig. This imposes a limitation on the design of the piston rod. The problem can be solved by butt joining together the piston rod and the bolt-shaped member at the final step after the piston rod has been assembled into the cylinder. If doing so, however, spatters (contaminants) caused by welding may adhere to the piston rod and damage the rod seal during use. Accordingly, the idea of carrying out the butt joining process at the final step has been given up on in the present state of the art.